The invention relates to a method for encoding digital data with at least two different degrees of error protection. That method comprises distributing uniform size user elements of the data over an array of at least two dimensions, supplementing according to at least one of the dimensions the user elements with first redundant elements of the uniform size of a first linear coding, and supplementing according to at least a second one of the dimensions the user elements and first redundant elements with second redundant elements of the uniform size of a second linear coding.
Such a method has been described in U.S. Pat. No. 4,680,764, which shows the error protection format of the so-called third layer in Compact Disc Read Only Memory, CD-ROM for short. The code is a pseudo-product code because the second redundant elements are not protected by the first error protective code. This means that the degree of error protection of the second redundant elements is less than the degree of error protection for either the user elements or the first redundant elements.
It has been found that often certain user elements have a higher importance than certain others, i.e., any interference of the former would cause relatively more damage than of the latter. For example, computer data is more vulnerable than audio, because errors in audio can be concealed, such as through interpolation that is useless for computer data. Also, audio is more vulnerable than video, because the human eye is more forgiving than the human ear. Also, among information of a single category, certain elements could warrant extra protective measures above those taken for other user elements. An example of elements justifying such extra protective measures would for example be intensity scaling factors of a video picture. The present invention therefore, carries the principle of different degrees of error protection to the user elements, and as such, scrupulously applies an input-output analysis of using greater redundancy versus realizing enhanced protection.